Scientists have been investigating ways of separating components of industrial gas streams for many years. Recently, with the climate changes being observed due to carbon dioxide emissions, ways of separating carbon dioxide from gas streams to try to reduce the impacts of global warming have been widely researched.
In general, gases such as carbon dioxide are separated from gas mixtures with, for example, methane, nitrogen and/or carbon monoxide by reversible absorption methods employing various chemical and/or physical solvents, or by reversible adsorption in beds of adsorbents (e.g. activated carbon). As conventional processes for treating carbon dioxide are highly energy consuming and depend on the use of additional chemicals, the cost as well as the increased demand for environmental protection enforce more efficient separation processes to evolve. Membrane technology is such a new separation technique. Membrane modules also significantly reduce weight and space requirements of separation equipment.
One option for membrane separation is the use of a facilitated transport membrane, also known as a supported liquid membrane (SLM) with mobile facilitated transport carriers. These have been studied for over two decades and are known to have both high permeability of gases and high selectivity. However, for the SLM membranes serious degradation problems, such as evaporation of solution and deactivation of complexing agent (carrier), have restricted their further development and application. Facilitated transport membranes with fixed carrier, such as the PVAm blend membrane claimed herein are therefore favoured.
Other alternatives to facilitated blend membranes are however known. In J. Membrane Science 163 (1999) 221-227, the separation and recovery of carbon dioxide is achieved using polyethyleneimine/polyvinylalcohol membranes. Such membranes are however very dense and they therefore possess poor permeance. In this regard, permeance is a measure of the flow of a gas through the membrane. High permeance implies the ability to separate large volumes of gas with a reduced membrane area.
In U.S. Pat. No. 4,690,766, a modified polysulphone semi permeable membrane is disclosed for use in reverse osmosis and ultrafiltration.
In EP-A-1900419 a mixture of PVAm and polyvinyl alcohol (PVA) is used as a gas separating membrane. The PVAm exemplified is of very low molecular weight and is thus used on a support with relatively low molecular weigh cut off (MWCO).
In WO2005/089907, a support coated with a cross-linked PVAm is used as a carbon dioxide separating membrane. The cross-linking agent ammonium fluoride is used to ensure the cross-linking occurs. This membrane suffers however, from a decrease in flux, in particular at higher pressures, when high molecular weight cut off (MWCO) porous supports, e.g. those of MWCO 50,000 or higher, are used. This is believed to be caused by a compaction of the selective membrane layer which may result in a “filling-in” of the pores of the support structure.
There remains a need therefore to design further gas separating membranes which do not suffer from the problems highlighted above but which also possess excellent target gas selectivity and high permeance. The inventors have surprisingly found that certain membranes, e.g. a gas separation membranes comprising polyvinylamine exhibit excellent separation properties, excellent mechanical properties and are very stable. Moreover, certain membranes do not suffer from pore blockages (filling in) which may occur with other supported membranes.
The inventors have made the following remarkable observations in relation to gas separation membranes:
1. That as an alternative to cross-linking using an external cross-linking agent, thermal treatment of the formed membrane can be beneficial to the permeance and selectivity of the membrane
2. That the use of a higher molecular weight PVAm polymer can improve permeance and selectivity of a membrane relative to lower molecular weight materials;
3. Using higher molecular weight PVAm polymers enables the use of higher MWCO supports which in turn can enhance the permeance and selectivity of the membrane;
4. That cross-linking the PVAm using an external cross-linking agent may reduce the permeance and selectivity of the membrane, especially for high molecular weight membranes which are usable without cross-linking;
5. That for some high molecular weight PVAm containing membranes neither cross-linking nor thermal treatment is needed at all;
6. That careful selection of the solvent used to cast the membrane can enhance permeance and selectivity of the membrane;
7. That control of the pH during casting of the membrane is critical to maximising permeance and selectivity of the membrane;
8. That pre-treatment of the PVAm polymer used to manufacture the membrane can improve the permeance and selectivity of the membrane;
9. That the use of a high carbon dioxide permeance polymer layer (such as polydimethylsiloxane, polyvinyl alcohol or chitosan layer) between the membrane and support gives the membrane much higher mechanical strength without damaging the selectivity and permeance of the membrane.
The inventive membranes of the invention may comprise at least one, preferably a combination of the inventive features mentioned above. The invention encompasses any (viable) combination of these features.